Exam Question

(a) The formation of NOx depends particularly on the temperature of the combustion. Highly efficient engines operate at a higher temperature and pressure than normal diesel engines. Higher the temperature higher the emissions of NOx (because more energy promotes the chemical reaction). These conditions favour the production of NOx gases. The quantity depends on the volume and duration of the hottest part of the flame.
(b) SCR (Selective Catalytic Reduction) is a method used to control NOx emission. This method involves injection of a fine mist of urea plus water (called as Diesel Exhaust Fluid - DEF) into the engine’s exhaust system to create a chemical reaction to turn NOx into Nitrogen and Water Vapour.
DEF is a non-hazardous solution, which is 32.5% urea and 67.5% de-ionised water.
The SCR system consists of a reactor, catalyst elements, soot blower, sensors, air reservoir, mixing devices, dosing unit urea injection nozzle, urea pump and safety control system.